Crop rotation is a fundamental disease management practice at PFR. PFR's rules for crop rotation are listed and described in Table 2. PFR does not follow a strict rotation plan. Rotation is managed spatially, temporally, and seasonally. Rotation is an effective disease management strategy for some crops and diseases but not for all.

According to Phil Foster (farmer) and Doug O'Brien (pest management consultant and scout), the disease management system (Table 1) has been mostly effective. Overall, yield and quality losses to disease have declined, with a few exceptions. These conclusions are supported by 30 years of experience, 25 years of yield and supplemental input records (1989-2014), and 17 years of scouting records (2-3 scouting visits per week, 1997-2014).

Diseases that have never occurred due to prevention (pathogen has never been introduced to farm on seeds, transplants, equipment):

White rot of onion and garlic (Sclerotium cepivorum)

Fusarium wilt of basil (Fusarium oxysporum f.sp. basilica)

Diseases that occurred regularly in the past and sometimes caused economic damage, but are no longer economically damaging:

Management strategies contributing to control are identified in parentheses after scientific name of pathogen. In some cases the causes of disease suppression are unknown.

PFR addresses disease problems through the use of multi-strategy toolboxes, many of which are continually evolving. Model disease-specific toolboxes are described below.

Verticillium Wilt (Verticillium dahliae) of Watermelon and Melon

Disease onset and trend: This disease occurs at the Santa Ana ranch in Hollister (SA) only. Prior to PFR's stewardship, conventional tomatoes and peppers were probably grown on this ranch and were likely the source of infestation. Severity is increasing (Fig 4.), and watermelon and cantaloupe yields have been decreasing dramatically (Fig. 5 and Fig. 6).

Crops affected: Watermelon, melon, potato, and eggplant. Galia and Sharlyn melons are very susceptible. Sweet potato is very susceptible but is no longer grown at PFR for this reason. Pepper and tomato are asymptomatic hosts (they are not affected by the disease but the pathogen grows on their tissues). Many other cash crops and vetch cover crops are hosts. Only a few crops are completely resistant: Sudan grass, oats, sweet corn, onion family crops, carrot family crops, and broccoli (McCain et al., 1981).

Management: While Verticillium propagules are very long-lived and rotation does not control this disease, PFR avoids planting extremely susceptible crops (or asymptomatic hosts) in succession. PFR has recently experimented with planting grafted watermelon transplants, which sometimes increase yields but do not seem to be effective when soil temperatures are high. Grafted eggplant transplants did not increase eggplant yields at PFR. O'Brien scouts susceptible crops, and PFR applies more frequent and lower-volume irrigations when crops show symptoms. There are no resistant watermelon varieties; however, some cantaloupe (Fig. 6) and canary melon varieties have some field resistance. Soil building does not seem to suppress disease development although it likely reduces crop stress and enhances root and overall crop growth. Broccoli, oats, and sudangrass pre-crops have been reported to suppress Verticillium wilt in subsequent crops, but this strategy has not proven effective at PFR. Reducing crop stress by reducing cucumber beetle feeding and damage might help increase yield, but cucumber beetles remain difficult to control.

2017 update: PFR purchased grafted melons (resistant rootstocks 'Carnivor' and 'Cobalt') in 2017. The rootstocks do reduce Verticillium damage and increase yields. However, these rootstocks are extremely susceptible to cucumber beetle larval feeding (the larvae hollow out the taproot). The rootstock raises yields in the first three watermelon blocks planted without cucumber beetle interventions, but the last two plantings have to be grown in a hoop house with sides and sticky tape barriers on the ends to keep the beetles out.

Fusarium Basal Rot (Fusarium oxysporum f. sp. cepae)

Disease onset and trend: Fusarium basal rot (FBR) has occurred at the Santa Ana ranch in Hollister (SA) since the mid 1990's, and was diagnosed for the first time at one San Juan Bautista ranch (SJB) in 2016. Prior to PFR's stewardship, susceptible onion varieties were probably grown on the SA ranch, and those were the likely source of infestation. Scouting reports show that severity is increasing (Fig. 7) and yields are decreasing (Fig. 8).

Crops: All of PFR's allium crops are susceptible, but shallots and red onions are the most susceptible.

Management: PFR has moved all shallots and some red onions to the disease-free San Juan Bautista ranches (SJB). However, not enough acreage exists at SJB to grow all the red onions, and more SJB ranches are likely to become infested. Only early red onions can be grown without yield loss. 'Mercury', a red onion with resistance to FBR, was grown until it was no longer commercially available in 2011. PFR bought a large quantity of seed in 2011 and grew it through 2013 while trialling and learning to grow new varieties. None of these new varieties have adequate resistance and yields have declined (Fig. 8). The onion family rotation rule (5 years allium-free) likely slows down the rate of increase in the number of pathogen propagules, but sometimes PFR cannot maintain a 5-year allium-free period, and disease continues to reduce yields. Onions receive overhead irrigation for the first few weeks and are drip-irrigated thereafter; drip supplies small amounts of water regularly and without water-logging to damaged onion roots. Thrips are managed to reduce crop stress. Soil building does not seem to suppress disease development although it likely reduces crop stress and enhances root and overall crop growth.

Downy Mildew of Cucumber (Pseudoperonospora cubensis)

Disease onset and trend: A new race of downy mildew appeared in 2009, reducing yields. Average yields increased thereafter. (Fig. 9).

Management: Starting in 2010, PFR reduced late-season acreage (August–October harvests), as downy mildew is most damaging during that period. One block of cucumbers is now grown in the greenhouse, as the greenhouse eliminates foliage wetness from rain and dew, and greenhouse cucumber yields are very high. Regalia® (extract of Reynoutria sachaliensis, Marrone Bio Innovations) is not applied to the greenhouse crops or to the first two spring field plantings, but for all later-season plantings it is applied at 14-day intervals from the 5-leaf stage until harvest. Copper-based materials have not shown efficacy.

Management: Crops are seasonally grown at the SA ranch to avoid the higher humidity at the SJB ranch (environmental/spatial rotation), and peppers receive overhead irrigation early in the season (before fruits touch the soil) to suppress early disease onset (overhead irrigation suppresses powdery mildews). Sulfur has been applied regularly since 1994 (sulfur figures, Figs. 11a and 11b), but only after powdery mildew was detected during field scouting, and mildew continued to significantly reduce yields (Fig. 10). Starting in 2007, PFR used scouting records to determine when first powdery mildew infections typically occurred in an effort to apply sulfur before first infection. After several years of early and thorough scouting and experimenting with the timing of sulfur applications, PFR now applies protective sulfur sprays very early in the season and has significantly increased yields. Planting succession crops upwind (spatial rotation for control of wind-blown pathogens) and other supplemental inputs have not shown efficacy.

Sclerotinia Drop of Lettuce and Celery (Sclerotinia minor)

Disease onset and trend: This disease was seen regularly in the early years but it is now economically unimportant.

Crops: Lettuce and celery.

Management: PFR aims for a three-year rotation for lettuce/celery crops (two years lettuce- and celery-free). Soil microbes supported by PFR's long history of soil-building may reduce sclerotia populations in the soil. In addition, soil-building likely reduces crop stress and enhances root and crop growth (lettuce yield, Fig. 12 and celery yield, Fig. 13).

Rust of Garlic and Leek (Puccinia allii)

Disease onset and trend: Garlic rust arrived in California and PFR in the mid-1990's and has damaged crops annually since that time. Garlic yields are highly variable and show an increase overall (Fig. 14), but PFR's yield goal of 400 boxes is rarely achieved due to rust-induced losses.

Management: Spatial rotation (wide separation and upwind planting of succession crops) is somewhat effective. Drip irrigation reduces leaf wetness. Rows are planted north to south. Regalia® is applied twice at a two-week interval beginning around the 4-leaf stage. Thereafter, sulfur is applied about every two weeks up until two weeks before harvest. Since 2010, garlic plantings have received from 6 to 13 sulfur applications (Fig. 15).

Prevention

Pathogen exclusion is one of the most reliable disease management strategies, and several very damaging diseases have never been introduced (on seed, transplants, or equipment) to PFR, including white rot of onion and garlic (Sclerotium rolfsii) and Fusarium wilt of basil (Fusarium oxysporum f. sp. basilica). PFR only buys seed, including garlic seed, from sources that screen for seedborne diseases.

Soil Building

PFR's intensive soil-building practices suppress damping-off of corn, bean, beet, and onion (Stone et al., 2004). However, the soil-building program does not appear to be contributing significantly to the control of other extremely damaging diseases such as Verticillium wilt and Fusarium basal rot, although it likely enhances root and crop growth and reduces plant stress.

Resistant Varieties

Genetic resistance is another of the most reliable disease management strategies. Selecting resistant varieties (pepper and tomato resistant to Verticillium wilt, zucchini resistant to viruses, and lettuce resistant to downy mildew) helps control what would otherwise be very economically damaging diseases at PFR. Fusarium basal rot of onion is controlled in white and yellow onions by genetic resistance, while there is no resistance in shallots. FBR was controlled to a significant degree in red onion crops by growing the resistant variety 'Mercury'. When 'Mercury' became commercially unavailable in 2012, red onion yields dropped. Red onions and shallots have an uncertain future at PFR. PFR has recently started applying sulfur very early to prevent infection of pepper, tomato, zucchini, winter squash, and carrots by powdery mildews. This method is management- and labor-intensive, and therefore expensive. Powdery mildew resistant varieties would be a far better strategy.

Crop Rotation

Crop rotation is the most reliable soilborne disease management strategy (Mohler and Johnson, 2009). PFR's rotation design priority is to manage critical soilborne diseases such as pink root and Fusarium basal rot of onion, Sclerotinia lettuce drop, and Phytophthora root rot of pepper. The diversity of PFR's crops and cover crops and its large scale provide PFR with more opportunities for temporal and spatial rotations than smaller and less diverse farms. The two geographically-separated ranches (Hollister/Santa Ana and San Juan Bautista) with different environmental conditions make it possible for PFR to rotate crops in space and to different environments (for example, moving an entire crop to another ranch for a period of time to avoid wind-blown pathogens, or to avoid temperatures that increase disease risk). This crop diversity has its costs—managing so many different crops and two widely-separated farms is expensive and requires exceptional management skills. Some crops are not well adapted to PFR's climate and soils, necessitating expensive inputs (such as plastic high tunnels and polyester row covers) to achieve adequate yields and acceptable quality. PFR needed to develop markets for non-shipping grade produce, causing increases in management and infrastructure costs.

Crop Scouting

Decades of regular scouting by Doug O'Brien, a PhD plant pathologist, along with diagnostic and management information support from university research and extension personnel, has helped PFR to rapidly respond to disease onset, to understand critical disease life cycles and the timing of disease onset, and to develop multi-strategy management toolboxes. In addition, O'Brien's years of scouting records and observations on how plant-growth-stage interacts with weather conditions led to preventive sulfur applications and much more effective control of powdery mildew in pepper, tomato, squashes, and carrots.

As described above, PFR hires an experienced crop consultant to scout its fields. In addition, PFR staff make all pesticide applications. This approach is very different from that of many large vegetable farms in the area, which hire integrated (scouting and application) pest management companies to provide these services. The pest management companies hire Pest Control Advisors (PCAs) to do the scouting and make pesticide recommendations. The recommended pesticides are then applied by the company applicators. Foster does not contract with a pest management company, as he thinks he gets more overall value from 1) hiring a highly-trained scout with a wide range of experience and ability who has no economic interest in whether or not a pesticide is applied; 2) making his own pest management decisions, informed by his many years of experience and input from the scout; 3) having his staff make the farm's relatively infrequent pesticide applications; and 4) absorbing both the risks and benefits of this approach. It takes years of experience and a good background in the science and practice of organic pest management to become an experienced scout, and there are not enough of them. O'Brien says that the development of technological systems that would allow him or other experienced organic scouts or PCAs to remotely observe multiple fields, while deploying lower-cost personnel and/or robotics in the fields on a frequent schedule, might result in better use of resources and improved management.

Supplemental Inputs

PFR applies no pesticides to most of its crops. However, some diseases cannot be managed without their use. Pesticides are used regularly for the management of powdery mildews; downy mildew of cucumber, onion and shallot; and garlic rust. Materials applied at PFR include sulfur products (for powdery mildews and garlic rust), Regalia® (extract of Reynoutria sachalinensis, Marrone Bio Innovations, for downy mildew of cucumber and garlic rust), and Cueva® (copper octanoate, Certis USA, for downy mildew of onion). PFR has experimented with other inputs allowed for use on organic farms, including Kaligreen® (potassium bicarbonate, Toagosei Co.), Serenade® (QST 713 strain of Bacillus subtilis; Agriquest), Sonata® (Bacillus pumilus strain QST 2808, Bayer), and neem-based materials, but these are not currently applied as the efficacy was low. Compost tea is applied regularly to many crops, but not for disease management, and no impact on severity of any disease has been observed.

IMPORTANT: Before using any pest control product in your organic farming system:

Read the label to be sure that the product is labeled for the crop and pest you intend to control, and make sure it is legal to use in the state, county, or other location where it will be applied.

Read and understand the safety precautions and application restrictions.

Make sure that the brand name product is listed in your Organic System Plan and approved by your USDA-approved certifier. If you are trying to deal with an unanticipated pest problem, get approval from your certifier before using a product that is not listed in your plan—doing otherwise may put your certification at risk.

Note that, although OMRI and WSDA lists are good places to identify potentially useful products, all products that you use must be approved by your certifier. For more information on how to determine whether a pest control product can be used on your farm, see the related article, Can I Use This Input On My Organic Farm?

The few failures of PFR's disease management system (garlic rust, downy mildew of cucumber, Verticillium wilt, Fusarium basal rot) indicate the need for new and/or better strategies. Abandoning crops when disease is uncontrollable (e.g., spinach and sweet potato already abandoned; garlic, late season watermelon, Galia and Sharlyn melons, cucumber and red onions under consideration) is always an option, but one with significant negative economic and farm system design implications.

Threats to large-acreage crops, such as onions (Fusarium basal rot), endanger the long-term sustainability of PFR's entire operation. Resistant varieties are urgently needed, and must be maintained in the long term. The red onion 'Mercury' was somewhat resistant to Fusarium basal rot, but the variety was discontinued. Many crops have been bred with economically successful resistance to downy mildew, rust, Verticillium, and Fusarium. Research to bring resistance to PFR's crops would solve problems, at least in the short term. Unfortunately, the development of resistant varieties for diseases for which conventional pesticides have been available has not historically been a priority for seed companies (e.g. downy mildew of cucumber); with the growth of the organic marketplace this is changing. Techniques such as steam and anaerobic disinfestations are expensive and management intensive, and unlikely to be implemented on a farm as large as PFR. Longer-term methods that anticipate pathogens overcoming resistance, such as moving the soil microbial community towards suppression (no evidence of effectiveness for some diseases), are also needed.

This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.